Michael McCarron, director of community affairs for the San Francisco International Airport, said about a dozen United flights had been affected at the airport Wednesday morning and most delays averaged 20 to 30 minutes. United is the airport's largest carrier.

"I expect it to decrease as the day goes on," he said. "And by noon, (it will) be pretty much back on track."

United's hubs are O'Hare, Dulles and airports in Denver, Los Angeles and San Francisco.